1. Field of the Invention
The present invention generally relates to the manufacturing of circuits in thin silicon wafers. More specifically, the present invention relates to the handling of such wafers.
The manufacturing of circuits is sometimes performed in particularly thin silicon wafers, for example, of a thickness smaller than 200 or 100 μm. Such wafers are very difficult to handle due to their sensitivity to shocks. To reduce risks of cleavage or breakage upon handling operations, it has been provided to fasten or glue the unprocessed surface of the wafer to another protection and handling wafer which will be called the support-wafer hereafter. The gluing is performed to enable subsequent separation of the wafer without damaging the wafer or the support wafer, to be able to use the support wafer again.
2. Discussion of the Related Art
Conventional sequences of separation of a wafer from a support wafer will be described hereafter in relation with FIGS. 1A, 1B, and 1C.
FIG. 1A illustrates, in a simplified cross-section view, a thin silicon wafer 1 glued by direct gluing on a support wafer 3 (FIG. 1B). The separation is here performed by means of a specific device 10 (FIG. 1C). The surface of wafer 1 glued to the support wafer is covered with a silicon oxide layer 2. Layer 2 generally is a specifically-deposited layer. Such a deposition is performed so that the external surface of layer 2 is substantially planar.
FIG. 1B illustrates, in a simplified cross-section view, an example of a support wafer 3. Support wafer 3 is a silicon wafer having dimensions identical to those of the wafer 1 that it is intended to receive. However, the thickness of support wafer 3, of at least 200 μm, is greater than that of wafer 1. The surface of support wafer 3 intended to receive wafer 1 includes a plurality of pads 4. Pads 4 are separated by intervals 5 which all communicate. Pads 4 have the same height and exhibit, at a macroscopic level, a substantially planar upper surface. The entire support wafer 3 (pads 4 and intervals 5) is covered with a silicon oxide layer 6. Layer 6 results either from a thermal oxidation of support wafer 3, or from the deposition of a specific layer.
Thin wafer 1 and support wafer 3 are glued by direct gluing and welded by an adhesion anneal. Then, wafer 1 is handled during the circuit forming via support wafer 3.
FIG. 1C schematically shows a cross-section view of a device 10 for separating thin wafer 1 from support wafer 3.
Device 10 appears in the form of a cylindrical box. The bottom of the box includes a horizontal plate 11 from which protrudes a vertical wall 12. A peripheral ring 13 having a substantially rectangular cross-section bears against plate 11. The planar upper surface of ring 13 supports a gasket 14. Plate 11 also supports a gasket 15.
As illustrated in FIG. 1C, upon separation, support wafer 3 and wafer 1 are altogether placed in separation device 10 so that support wafer 3 is on gasket 15.
Separation device 10 is then closed back by a cover 16. Cover 16 includes a peripheral ring 17 protruding to the outside of device 10. Ring 17 is similar to ring 13 and contacts gasket 14. The central portion of cover 16 supports a gasket 18. Gasket 18 is placed so that it bears against wafer 1 and support wafer 3 altogether above gasket 15. Cover 16 is aligned and locked by the screwing of several screws 19 distributed on its periphery, and passing through ring 17 to screw in ring 13.
Cover 16 being closed, the separating of the assembly of wafer 1 and of support wafer 3 is performed by injecting gas between silicon oxide layers 2 and 6. Such a separating has many disadvantages.
A first disadvantage is that such a separation imposes constraints on the roughnesses of layers 2 and 6. Indeed, the smaller the roughness of layers 2 and 6, the greater the adherence between wafer 1 and support wafer 3. Under a given roughness, the separation becomes difficult (or even impossible) and requires pressures and injection flows which break wafer 1 and support wafer 3. A compromise must thus be made between sufficiently rough layers 2 and 6 to ensure sufficient adherence to enable handling of the assembly, while maintaining a sufficient roughness for the separation to be possible in reasonable time and pressure conditions.
Another disadvantage of the separation is that it imposes limits on the thermal processings of the assembly. Indeed, the adherence of layers 2 and 6 is increased upon high-temperature anneals. The anneal durations and temperatures must thus be limited for the separation to still be possible. Such limitations of the anneal conditions do not enable forming any circuit in thin wafers.
Another disadvantage is the necessary use of human intervention to monitor or perform the screwing of cover 16. Indeed, the pressure exerted by gaskets 15 and 18 on wafer 1 must be controlled, upon closing of cover 16, since, in case of too high or too low a pressure, wafer 1 risks being damaged upon separation. Indeed, under the effect of the injected gases, it tends to be slightly lifted with respect to support wafer 3. If gasket 18 is too crushed against wafer 1, it cannot damp (absorb) this lifting. Then, wafer 1 deforms, cleaves and breaks. Conversely, if gasket 18 is insufficiently compressed against wafer 1, upon lifting of wafer 1, either said wafer can be projected against gasket 18, or a lateral clearance can appear and wafer 1 may hit against the walls of device 10. Further, the thickness of the assembly of wafer 1 and of support wafer 3 is not known and may slightly vary. Further, gaskets 18 and 15 may be partially damaged (crushed).
Another disadvantage is the need to control the rate of inflow of the separation gas. Indeed, if the flow is high, the unglued portions of wafer 1 are abruptly raised and tear the portions glued to pads 4. The gas injection flow must thus be progressively increased. Further, if the gas injection is maintained at a high flow at the end of the separation, an abrupt upraising of wafer 1 may be caused, which results in risks of shocks against gasket 18. These injection control constraints further complicate and slow down the separation process.